The present invention relates to a transmission to transmit a drive power for an automotive vehicle and the like, and particularly a twin-clutch transmission in which two clutch mechanisms are disposed on an input shaft closely to each other.
Conventionally, the twin-clutch transmission with two clutch mechanisms disposed on the input shaft, which performs a prompt shift control by switching the clutch mechanisms in such a manner that when one clutch mechanism is engaged, the other unengaged clutch mechanism performs the shift, is known.
For example, U.S. Pat. No. 4,463,621 discloses this kind of twin-clutch transmission. In the twin-clutch transmission, two countershafts are required in order to provide two drive transmission paths from the input shaft to the output shaft, as disclosed in the above U.S. patent document. However, providing two countershafts separately in the transmission may cause a large-sized apparatus. As a result, there is a problem that the transmission could not be located properly in the tunnel portion of the vehicle floor.
Then, another structure in which the two countershafts are disposed coaxially, as disclosed in U.S. Pat. No. 6,460,425, is considered. Namely, a first countershaft is formed of a cylindrical member and a second countershaft is formed of a shaft member, and these two countershafts are disposed coaxially. This coaxial disposition can provide a compact twin-clutch transmission, solving the above-described problem.
Herein, the transmission is designed so as to bear the drive torque properly for transmitting the drive torque from the drive resource such as an engine to the driven wheels. Thus, the diameter of any shafts is also designed properly. Also, in general, the largest drive torque (hereinafter, referred to as “the maximum transmission torque”) is applied to the transmission when the drive torque is transmitted to the wheels via the forward lowermost speed ratio during a vehicle starting. Accordingly, the diameter of the shaft carrying this forward lowermost-speed-ratio gear is required to be designed properly so as to bear this maximum transmission torque.
Namely, in the twin-clutch transmission disclosed in the above-described latter U.S. patent document, the first-speed-ratio counter gear, as the forward lowermost-speed-ratio gear, is carried by the second countershaft which is inserted into the first countershaft. Thus, the diameter of the second countershaft should be large enough to bear the maximum transmission torque.
However, providing such a large shaft of the second countershaft may necessarily cause the large shaft diameter of the first countershaft and the large diameter of the gears carried by the first countershaft. Such a large gear diameter may also require the large distance between shafts to ensure a proper gear ratio, resulting in a large-sized transmission.
Accordingly, the structure of the latter U.S. patent document has a problem that it could not properly obtain an advantage of making the twin-clutch transmission compact by disposing the two countershafts coaxially.